The present invention relates generally to fluid drop systems for use in aircraft. More particularly, the invention concerns such a system that includes a fluid-tillable tank having openable/closeable door panels and a fluid volume sensor, an operator's panel for selecting a drop rate and volume and a controller responsive to the sensor and the panel for controllably opening and closing the door panels to release the fluid, e.g. for fire-fighting. An alternative system further includes an aircraft groundspeed sensor and the controller also is responsive thereto yet to control fluid release in relation to actual groundspeed.
Known aircraft fluid drop systems control door panels by incrementally opening them to release fluid, based upon the level of fluid remaining within the tank. Due to numerous unaccounted for variables, e.g. mechanical linkages and their associated response delays, fluid inertia at the start of drops, and fluid mass changes, fluid sloshing within the tank and changes in the groundspeed of the aircraft during drops, known drop systems are notoriously incapable of accurately controlling the fluid drop rate. In demanding applications such as firefighting, such inaccuracies result in great inefficiencies in terms of ground coverage by the very limited volume of water and fire retardant mixture that can be airlifted by such aircraft. Variation in aircraft groundspeed is entirely unaccommodated by prior art systems and is exacerbated by the increasing use of helicopters, which are capable of turning and banking relatively quickly to avoid hazards such as intense heat, trees and utilities.
The use of fluid level or pressure (head) within the tank as a basis for rate controlling the release of fluid is particularly susceptible to producing inaccurate fluid drop rates. Primarily, this is because level or head control assumes that the behavior of fluid in the tank is static. In fact, fluid within the tank is subject to many dynamic forces including inertia; acceleration due to gravity; fluid sloshing; aircraft acceleration; and mechanical linkages and their relatively slow responses to control stimuli. Such forces are variable and generally act upon the fluid release system throughout its operation even during a fractional tank volume fluid drop. One seemingly desirable solution to the problem would be to use a flow rate sensor associated with the tank, but such is not possible due to the relatively long openings, e.g. more than eight feet, defined by the door panels of known drop tanks.
The present invention in its preferred embodiment uses sophisticated electronic control circuitry to rate control fluid drops based on fluid volume. It does so by constant comparison of the instantaneous volume of fluid remaining in the tank with a predetermined, desired instantaneous remnant volume in accordance with a defined, declining volume profile. In accordance with the preferred embodiment described and illustrated herein, the operator panel permits a user to select one of three different, constant drop rates, e.g. 75, 300 or 700 gallons/second (gps), and one of four different drop fractions, e.g. 25, 50, 75, or 100%. Of course, more, fewer or different drop profiles, rates and fractions, within the spirit of the invention, readily may be accommodated.
These and additional objects and advantages of the present invention will be more readily understood after a consideration of the drawings and the detailed description of the preferred embodiment.